Houston's mayor kept facing questions Tuesday about his decision not to order an evacuation of the notoriously flood-prone city ahead of Harvey's arrival, even as overflowing reservoirs led several suburbs to move people out.

Instead, Mayor Sylvester Turner remained resolute in his advice to residents since the storm made landfall on Friday: hunker down at home.

Massive flooding from Harvey forced thousands of rescues that overwhelmed emergency responders. The George R. Brown Convention Center nearly doubled its expected capacity of 5,000, with people seeking refuge from the waist-deep waters that had neighborhoods resembling lakes.

"I want to say this again, because I guess it's been missed, but you cannot evacuate 6.5 million people within two days," Turner said Tuesday, referring to both the city and its surrounding areas. "That would be chaotic. We would be putting people in more harm's way."

The situation is reminiscent of Hurricane Katrina in 2005, when New Orleans Mayor Ray Nagin and Louisiana Gov. Kathleen Blanco both drew criticism for not ordering an evacuation sooner. Nagin did so one day before landfall, and levee failures led to disastrous flooding that plunged the city into chaos for days.

Experts said evacuating during a hurricane is a complicated decision with major ramifications, and none who spoke to the Associated Press second-guessed Turner. Harvey intensified quickly into a Category 4 hurricane on Friday, leaving the mayor and others a tight timeframe to safely move out a large number of people.

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"This is all the information that's coming into the mayor, and he's got to go, 'Oh my God, what am I going to do?'" said Susan Cutter, the director of the Hazards and Vulnerability Research Institute at the University of South Carolina. "They're damned if they do, damned if they don't."

Questions started Friday when Texas Gov. Greg Abbott suggested Houston residents get out while they could. At the time, Harvey was a powerful hurricane hours from landfall about 175 miles to the south near Corpus Christi. But the sprawling storm's outer bands promised heavy rains for Houston, long susceptible to flooding because of its flat terrain and runaway development that has paved over water-absorbing wetlands.

Hours after Abbott's remarks, Turner and other local leaders cautioned people to think twice before fleeing unnecessarily, especially when other communities more directly in the path of Harvey needed the same routes.

Once Harvey's rains turned torrential in Houston over the weekend and made conditions too dangerous for leaving, some residents began doubting Turner's decision.

Experts said Harvey presented a uniquely difficult decision for Houston officials because evacuations aren't typically done for flash-flooding from rains, but rather from storm surge concerns in less populated areas closest to the shore.

A study by the state of South Carolina this year estimated that the time needed to evacuate the Charleston area ahead of a similarly sized hurricane would be 27 to 30 hours at its fastest, if traffic lanes in both directions were used.

Moving people from an inland urban center like Houston — the city alone is about three times larger than Charleston — would require at least 36 to 48 hours, experts said. But Harvey took only 56 hours to intensify to a Category 4 hurricane.

Harvey strengthened into a hurricane by noon Thursday. Hours later, the National Hurricane Center predicted "devastating flooding" from the strengthening storm. The center started warning of "catastrophic flooding" across southern and southeastern Texas Friday morning, and the storm made landfall by 10 p.m.

"The problem with evacuations is, when do you call them?" said George Haddow, a former deputy chief of staff at the Federal Emergency Management Agency who rode out hurricanes with his family in New Orleans. "You don't want to call an evacuation so early and the storms take a left and hits Galveston."

Ordering an evacuation, experts said, would've come with its own risks, such as highways clogged with motorists caught in severe thunderstorms or, worse, tornadoes that Harvey's outer bands might cause.

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"I don't think they ever would have imagined having to evacuate the whole community in that amount of time," said Joe Myers, a former top emergency management director in North Carolina and Florida. "I think they did the best they could do under the situation."

In defending his decision, Turner has repeatedly invoked grim memories of Hurricane Rita in 2005, when problems from an evacuation of 3 million — roughly half of greater Houston — were blamed in many of the more than 100 deaths reported.

Massive traffic congestion toward Austin, San Antonio and Dallas caused delays of as long as 20 hours and even stranded out-of-gas motorists as Rita approached the Gulf Coast. In the worst incident, 23 nursing home patients were killed when the bus transporting them to North Texas exploded.

The region overhauled its evacuation plan after Rita and there was far less trouble when a quarter-million people were ordered out ahead of Hurricane Ike in 2008. Even so, Turner said he characterized a mass evacuation as a greater danger, especially the city wasn't in Harvey's direct path.

"You cannot put 6.5 million people on the road two days before a storm that you don't know where it is going," he said Tuesday. "It is absurd."

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But, some experts said, the option didn't need to be binary between the entire metro area staying or going.

A brief summary of the emergency operations plan for the county that is home to Houston covers evacuation of "all or any part," if that is determined the most effective way "for protecting the population." A full copy of the plan was not immediately available, and officials did not respond to requests.

The county's plan raises questions about whether targeted evacuations of susceptible neighborhoods were considered, even if only to higher ground in multi-story buildings, and possible to execute.

"It's really an impossible dilemma he was put into," said Shirley Laska, past director of the Center for Hazards Assessment, Response and Technology at the University of New Orleans. "But it shouldn't have to be groundless, and in the future, it won't be because of how horrible the event is to Houston."